Japanese Patent Application Publication No. 2011-19338 (JP 2011-19338 A) discloses a DC-DC converter. This DC-DC converter includes a DC-DC converter circuit including a plurality of switching elements connected in parallel to each other, a controller that controls operations of the plurality of switching elements, and a plurality of temperature sensors connected to the controller for measuring respective temperatures of the plurality of switching elements. The controller drives only a part of the plurality of switching elements to start control of the DC-DC converter circuit, and increases a number of switching elements to be driven when the measured temperature of the switching element being driven exceeds a threshold. According to such a configuration, the number of switching elements to be driven can be changed according to a magnitude of a load, thus losses in the switching elements (for example, a switching loss) can be reduced as compared to a configuration that always drives all of the switching elements.
In the aforementioned DC-DC converter, since only a part of the plurality of snitching elements may be driven in a certain situation, differences will be generated in use frequencies among the plurality of switching elements. Normally, deteriorations of a switching element and its peripheral structure (which will hereinbelow be simply termed “deterioration of switching element”) progress quicker with higher use frequency of the switching element. Thus, if differences are generated in the use frequencies among the plurality of switching elements, the deterioration may progress earlier in some of the switching elements, resulting in an early termination of product life for the DC-DC converter despite the deterioration of another switching element being within an allowable range. In regards to this, in the DC-DC converter of JP 2011-19338 A, the switching element to be driven is randomly or periodically changed upon driving a part of the plurality of switching elements, and this enables the plurality of switching elements to be used at a same frequency.